bridge: catch invalid stp state

bridge: catch invalid stp state

The stp state parsing was putting result in an __u8 which
would mean that check for invalid string was never happening.

Caught by enabling -Wextra:
    CC       mst.o
mst.c: In function ‘mst_set’:
mst.c:217:27: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  217 |                 if (state == -1) {

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>